Advanced airway assessment techniques

Key points.

• •The sensitivity and positive predictive value of traditional airway assessment tools are poor at predicting the difficult airway.
• •Advanced airway assessment techniques exist, which are more informative and relatively easy to learn and interpret.
• •Nasoendoscopy can be used in both the elective and emergency setting to assess supraglottic and glottic pathology.
• •Virtual endoscopy uses pre-existing CT or MRI images to simulate the views seen during flexible nasoendoscopy.
• •The use of ultrasound in airway assessment is increasing; it offers additional real-time information and aids decision making.

Learning objectives.

By reading this article you should be able to:

• •Distinguish which advanced assessment technique may be used in differing situations.
• •Describe the use of nasoendoscopy for airway assessment.
• •Specify the role of virtual endoscopy in special situations.
• •Discuss the use and limitations of ultrasound for airway assessment.

Introduction

Patients with a known or suspected difficult airway presenting for elective or emergency surgery may pose a significant challenge to anaesthetists.

Traditional preoperative airway assessment comprising bedside examinations and tests lack the sensitivity to predict difficult facemask ventilation, laryngoscopy and tracheal intubation. Evidence also suggests deficiencies in the ability of anaesthetists to predict airway management difficulties and pathology, and plan subsequent airway management.¹ An analysis of closed claims of the management of difficult tracheal intubation in the USA found that 76% of patients had preoperative predictors of difficult tracheal intubation. Three-quarters of the claims involved judgement failures, including a lack of proper airway management plan. These judgement failures may result in serious complications including death and hypoxic brain damage.²

The 4th National Audit Project of the Royal College of Anaesthetists and the Difficult Airway Society (NAP4) highlighted instances in which further assessment of patients with imaging, nasoendoscopy, or both would have been beneficial, particularly in those who were considered at high risk of a ‘can't intubate, can't oxygenate’ (CICO) situation.³

Potential difficulty with facemask ventilation, laryngoscopy and tracheal intubation may be underestimated in patients with pathology of the tongue, epiglottis, glottic opening or larynx. These are not always addressed during the usual bedside airway assessments. Presence or absence of signs and symptoms may be unreliable indicators of the significance of the lesion. Forty-three of the 133 serious airway complications reported in NAP4 were in patients with head and neck pathology, and almost three-quarters of cases involving emergency front-of-neck access came from this group.³ Rosenstock and colleagues⁴ also found that 85% of their emergency surgical airways occurred in patients with known airway pathology or undergoing head and neck surgery.

The positive predictive value of bedside tests is limited. Formal investigations such as X-ray, CT and MRI scanning may all provide additional information about a potentially difficult airway. These modalities, however, provide a static snapshot of the airway anatomy, often in awake, supine patients and do not represent the dynamic changes seen after induction of anaesthesia or positional changes. Such investigations are not appropriate in all patients and must be interpreted correctly. In selected high-risk patients, advanced airway assessment techniques such as nasoendoscopy, virtual endoscopy (VE) and ultrasound can be useful when planning a safe airway strategy.

Nasoendoscopy

Flexible nasoendoscopy (FNE) involves using a narrow (2.7–3.5 mm) flexible endoscope to visualise the nasal passages, pharynx and larynx. Ear, nose and throat (ENT) specialists often perform nasoendoscopy in outpatient and emergency department settings to assess the posterior nasal space, oropharynx and glottis. It enables an objective, contemporaneous assessment of the airway, whilst demonstrating the possible impact of pathology and location of any obstruction. Flexible nasoendoscopy is underused in anaesthesia despite there being a recommendation in NAP4 to consider it before induction of general anaesthesia in patients with neck masses and stridor. Flexible nasoendoscopy may increase patient safety by providing the anaesthetist with superior anatomical information. It allows assessment of whether the pathology may preclude face mask ventilation (e.g. extensive supraglottic oedema), prevent direct laryngoscopy (e.g. a base of tongue tumour) or impede successful tracheal tube placement (e.g. secondary to a mass or lateral displacement of the larynx). Flexible nasoendoscopy can therefore increase the clinician's confidence in formulating a safe airway management plan.

Elective setting

Nasoendoscopy can be performed rapidly in the preoperative assessment clinic, ward or in the operating theatre. Flexible nasoendoscopy assesses the supraglottic and glottic airway, including vocal cords and their movement and both piriform sinuses and provides useful, up-to-date information regarding upper airway anatomy and any airway pathology (Fig. 1A). After topicalisation of the airway, the subglottic anatomy can also be evaluated. Recording the examination serves to document the patient's preoperative airway assessment.

Fig 1.

(A) Left: normal epiglottis view from nasoendoscopy. (B) Right: abnormal epiglottis, view of acute epiglottitis from nasoendoscopy.

Rosenblatt and colleagues evaluated the use of FNE by documenting the proposed airway plans before nasoendoscopy and induction.⁵ Anonymised FNE recordings were subsequently reviewed and compared with the airway management plans. Using preoperative FNE changed the planned airway management in 26% of patients.⁵ Preoperative FNE with tongue protusion has also been shown to improve the prediction of the modified Cormack Lehane grade.⁶

Emergency setting

Owing to the risk of laryngospasm, complete airway obstruction, or both, extreme caution is required whilst performing nasoendoscopy in emergency clinical situations such as patients with stridor and suspected epiglottitis (Fig. 1B, Fig. 1 online Video 1). If nasoendoscopy is to be performed in such situations, then it should be undertaken by an experienced clinician, which may be an ENT surgeon. Mucosal injury, even from minor trauma, can result in epistaxis and significant airway bleeding in patients, and particular caution should be taken in those with known impaired clotting. Craniofacial trauma is considered a relative contraindication to FNE because of the risk of intracranial passage of the nasoendoscope.

Limitations

It must be remembered that the FNE view after induction of general anaesthesia may differ from the view in a conscious patient, where the patient maintains pharyngeal tone and is in a semirecumbent position. This must therefore be taken into consideration when devising the plan for airway management. Significant reductions in pharyngeal tone may also lead to airway obstruction. Laryngospasm is a known complication in elective situations, and although usually self-limiting it may cause airway obstruction and oxygen desaturation.⁷

Overview of the technique

A trained clinician, specialist equipment, an appropriate clinical setting and a compliant patient are all required. Emergency equipment, drugs and appropriate monitoring should be available. The patient should be sitting in a ‘sniffing the morning air’ or semi-recumbent position. As the nares are not always symmetrical or fully patent, patency should be assessed before passing the nasoendoscope to minimise discomfort and potential trauma. This can be done by asking the patient to inhale or sniff through their nose whilst obstructing the contralateral nostril. The endoscope should be orientated, focused, light sourced checked, white balanced and lubricated, ensuring the tip is clear. Touching the end of the nasoendoscope on the tongue for a few seconds warms the tip and reduces the risk of blurred or suboptimal images caused by fogging. Topicalisation of the nasopharyngeal passages with local anaesthetic and vasoconstrictor may aid patient comfort in addition to reducing the risk of bleeding. The nasoendoscope can be passed carefully through the nostril, observing the anatomy of the nasopharynx down to the glottis, assessing the upper airway to provide useful information on the impact of any pathology which may be present. If required, the subglottic anatomy can also be assessed but additional topicalisation may be required for the patient's comfort and tolerance of the procedure.

Ultrasound

Point-of-care ultrasound (POCUS) of the airway is a rapid, non-invasive technique which can be performed at the bedside. Abernethy and colleagues used US to assess tongue movement during anaesthetic induction in the early 1990s.⁸ Since then, technology, and the images produced, have markedly improved. Airway US is easy to learn and perform, with trainees becoming competent in a relatively short period of time.^9,10

Point-of-care ultrasound can be used at various stages of airway management, from airway assessment to confirming the position of the tracheal tube, and readiness for extubation. Studies comparing ultrasound with CT scanning have shown that upper airway structures can be reliably visualised and assessed with ultrasound.¹¹ An ultrasound machine with both a high frequency linear and a curvilinear probe are sufficient to scan the majority of airway structures.

Clinical scenario 1.

A 34-year-old primigravida presented at 35-weeks gestation, not in labour but with a hoarse voice and inspiratory stridor. The patient herself was an ex-premature delivery at 26 weeks, which had led to prolonged intubation with resulting subglottic stenosis and a subsequent tracheostomy until 6 years of age. Following multiple tracheal reconstructions, she had been lost to follow up since the removal of granulation tissue 13 years previously. She reported worsening dyspnoea throughout the pregnancy with a maximum exercise tolerance of 1 flight of stairs.

Airway examination revealed previous surgical and tracheostomy scars anteriorly and reduced neck extension, but otherwise the airway assessment was unremarkable. She had a whispering voice with biphasic stridor.

CT imaging showed diffuse glottic and sub-glottic stenosis, with a diameter of 6mm. The stenosis extended 3cm distal to the glottis.

A multidisciplinary team meeting was convened to discuss her peripartum management. CT images were imported into the 3-D endoscopy software package and a virtual fly-through was created. (Fig 4) (Supplementary Video 3) The VE reconstructions revealed facemask ventilation and asleep oral intubation with direct laryngoscopy was not likely to be possible, and tracheostomy under local anaesthesia would be difficult.

The decision was made to deliver via elective caesarean delivery under a low-dose combined spinal epidural (CSE) anaesthesia technique with the presence of an ENT surgeon and anaesthetist comfortable in performing awake tracheal intubations if required.

Following a CSE in the sitting position, she was given supplementary humidified oxygen via nasal high flow cannulae at 50 litres/minute. She was able to tolerate lying supine with 15 degree left lateral tilt until delivery. There was no requirement for general anaesthesia or airway augmentation. There was an uncomplicated and uneventful delivery of a healthy baby and she was followed up by the ENT specialists.

Preoperative airway assessment

Tongue thickness may affect the ease of direct laryngoscopy and intubation, and it can be measured using a curvilinear US probe in the submental, median sagittal plane. A tongue thickness >6.1 cm, and a tongue thickness to thyromental distance ratio >0.87 have been shown to be independent predictors of difficult tracheal intubation.¹² The vocal cords and adjacent structures can be visualised with the linear probe placed transversely at the thyrohyoid window (Fig. 2). This technique has been used in the perioperative assessment of vocal cord movements in patients undergoing thyroidectomy.¹³

Fig 2.

Transverse ultrasound image of vocal cords and probe placement. AC, anterior commissure; A, arytenoid; VC, vocal cord.

Tracheal tube size

Point-of-care ultrasound can also be used to assess subglottic diameter of the trachea. This can be performed in children to estimate the tracheal tube size required, or in adults to demonstrate any degree of tracheal deviation and stenosis. To measure the width and location of the underlying trachea, place a linear probe in a transverse mid-tracheal position (Supplementary Fig. S1). This is superior to age- and height-based formulae in estimating tracheal tube size in children, although there is variability between users.^14,15

Cricothyroid membrane

Airway POCUS is often used to mark the location of the cricothyroid membrane (CTM) before anaesthesia. This can be performed in a known or suspected difficult airway in preparation for potential cricothyroidotomy, should a CICO scenario occur. Both transverse and sagittal scans have significantly higher success rates in identifying the CTM than traditional palpation and landmark techniques¹⁶ (Fig. 3A and B). This has also been shown in patients with a high BMI and in pregnancy.¹⁷ When visualising the CTM in the sagittal plane the tracheal rings resemble a string of pearls (Fig. 3C).

Fig 4.

Virtual endoscopy image of case showing the level of the stenosis with pink marker denoting level of fly-through. (A) Axial plane. (B) Coronal plane. (C) Sagittal plane. (D) Reconstructed fly-through.

Fig 3.

(A) Left: transverse ultrasound image of trachea showing cricoid ring (C) and trachea (T). (B) Centre: sagittal ultrasound image of tracheal showing the cricoid ring (C) cricothyroid membrane (CTM) and thyroid cartilage (TC). (C) Right: sagittal ultrasound image of tracheal rings showing cricoid ring (C) trachea (T) and tracheal rings (TR).

Fasting status

Aspiration is the leading cause of airway-related mortality and morbidity in patients under general anaesthesia.³ Bedside gastric POCUS is a rapid, non-invasive technique to estimate the volume of gastric fluid in adult and paediatric patients and has been discussed recently in this journal.¹⁸

Assessment of tracheal intubation

Confirmation of tracheal tube placement with waveform capnography is the gold standard. Although POCUS has a high diagnostic value for identifying oesophageal intubation before attempting ventilation, it should not be used as an alternative to capnography.^19,20 To visualise the oesophagus, the probe can be used in transverse plane at the level of the second and third tracheal rings (Supplementary Fig. S2; Fig. 2 online video).

Point-of-care ultrasound can also be used for determining the correct depth of tracheal tube placement. Although not standard practice, Tessaro and colleagues found a high sensitivity and specificity using ultrasound and saline filled cuffs in children using the tracheal rapid ultrasound saline technique (TRUST).²¹

Vocal cord assessment

Vocal cord function may be assessed using US. Laryngeal cartilage and vocal cord movement can be visualised using a linear probe placed transversely over the thyroid cartilage in a spontaneously breathing patient before extubation. This allows assessment of superior and right recurrent laryngeal nerve function after thyroid surgery.²²

Tracheostomy procedures

Identification of the trachea can be challenging in patients who are obese, have neck masses, or have had previous surgery or radiotherapy to the head and neck. Point-of-care ultrasound can identify the tracheal position, depth and the presence of any overlying vessels at the proposed insertion site for percutaneous or surgical tracheostomy. Ultrasound also aids in the selection of the best tracheal ring interspace for the insertion, whilst providing information on the size and type of tracheostomy tube required.²³

Whilst POCUS for airway management continues to develop, more research is required to demonstrate its effectiveness. Scans are easy to perform, but results are operator-dependent and require a cooperative patient and adequate time. A limitation of ultrasound imaging is that only anterior structures can be directly visualised as ultrasound waves cannot travel through air.

Virtual endoscopy

Virtual endoscopy is the process by which CT and MRI images can be used to create a (3-D) reconstruction. This technology has been used to assist in pathological diagnoses and guide biopsies by radiologists and bronchoscopists.^24,25 However, VE can be used by anaesthetists to assess and plan the airway management of patients with difficult airways.²⁶ This is done by importing the digital imaging and communications in medicine (DICOM) data files to a 3-D endoscopy software package, which in turn constructs a flight pathway along the patient's airway. A virtual camera may then be advanced and manoeuvred to simulate views seen during flexible endoscopy (Supplementary Fig. S3). These images can be created before surgery, thereby enabling the anaesthetist to plan an airway management strategy and a safe path for the endoscope, if this is the chosen technique.

An advantage of VE is that it provides a non-invasive, anatomically accurate representation of the patient's airway. The 3-D ‘fly-through’ video can be viewed simultaneously with CT/MRI images, which may improve interpretation of the anatomy, traditionally seen in solely conventional two-dimensional format.

El-Boghdadly and colleagues found that adding VE to airway assessment led to changes in airway management in 48% of cases, with 90% of these changing to a more cautious plan.²⁷ There was a significant increase in the diagnostic accuracy for supraglottic, glottic and infraglottic lesions, suggesting that this can lead to safer airway management strategies in patients with head and neck pathology.²⁷

Limitations

The quality of the fly-through videos are only as good as the quality of the uploaded DICOM data files. Small lesions may be missed, natural colour is absent, secretions can be incorrectly reconstructed as lesions and excessive smoothing of the airway walls can occur. This is an elective assessment using pre-existing CT scans and should not delay the management of an acutely obstructed airway. Unlike nasoendoscopy or POCUS assessments, VE is not a contemporaneous, dynamic examination of the airway.

Patients who present with head and neck pathology are more likely to have difficult airways and require an emergency surgical airway if difficulties arise; therefore, detailed planning can help reduce the risks.⁴ Some of the applications of these advanced airway assessment techniques are demonstrated using the two clinical scenarios.

Conclusions

A range of techniques are available to anaesthetists to aid with assessment of the potential difficult airway. There is an increasing body of evidence to suggest they should be considered in order to aid planning and maximise patient safety.

Nasoendoscopy can be safely and easily performed before anaesthesia and surgery; although it does not guarantee an ability to successfully intubate the trachea it is a useful adjunct to the anaesthetist's armamentarium. Upper airway POCUS aids airway management because of its ease, accessibility and lack of invasiveness. The addition of VE to the assessment may aid the development of a safer airway management plan in high-risk patients. By providing a virtual fly-through of the best route for intubation with a flexible bronchoscope, VE potentially reduces the failure rate of the technique.

Declaration of interests

IA is honorary secretary of the Difficult Airway Society, and has received honoraria from Ambu, Verathon Medical and Fisher & Paykel for travel and accommodation to attend meetings. No other external funding or competing interests declared.

Biographies

James O'Carroll FRCA is a specialty registrar in anaesthesia at Guy's and St Thomas' NHS Foundation Trust. He has undertaken fellowships in airway, liver transplantation and hepatobiliary anaesthesia.

Yasmin Endlich FANZCA MMed is a consultant anaesthetist working across the Royal Adelaide and Women's and Children's Hospitals in South Australia. She is a senior clinical lecturer at the University of Adelaide and the current chair of the Australian and New Zealand Airway Special Interest Group. She has presented on point-of-care ultrasonography (POCUS) courses for airway management nationally and internationally.

Imran Ahmad FRCA is airway lead and consultant anaesthetist at Guy's and St Thomas' NHS Foundation Trust, London. He is honorary clinical lecturer at King's College London and honorary secretary of the Difficult Airway Society. He is an author of the Difficult Airway Society's guideline on awake tracheal intubation.

Matrix codes: 1C01, 2A03, 3A01

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Appendix A. Supplementary data

The following are the Supplementary data to this article:

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